Pure CSS3, no javascript. Webkit browsers get a little bonus animation. The :target pseudo selector is truly a powerful new feature in CSS3.
Targeting a link directly to the URL with the hash tag will trigger the "LighterBox" onload. http://www.htmlstack.com/lighterbox/#img1

11-30-2010: Pretty basic at this point, but should be able to add in the standard lightbox features we have come to know and love.

Another variation here